Joint Tactical Information Distribution System

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While it is often used generically to refer to the main tactical information system among the U.S. and its allies, the Joint Tactical Information Distribution System (JTIDS) is actually the infrastructure below the voice and data information of the various applications that run over it. To say, however, that a given aircraft, computer, or radio is "JTIDS-compatible", especially "JTIDS Link 16", means, in practice, that it can be a full partner in information exchanges. JTIDS Link 16 compatibility is a first-phase objective for the Joint Tactical Radio System (JTRS).[1]

Each participant receives data on the overall tactical situation. This, minimally, consists of the participant's location, course and speed, originally determined by the TACAN system, and, today, may be from GPS. Other messages sent by participants include target acquisition data, the status of the platform's weapons, which is compiled from position location, TACAN navigation and target acquisition data generated by the platform and one or more other linked platforms.

Given that all subscribers to the net are able to supply such information, each platform can determine what targets are covered or not covered and will know whether other friendly aircraft or ground forces are available for support.

Membership, or non-membership, is itself a form of information. If every platform assigned to a target is in the net, and there is a list of targets available to the receiver, the absence of an assignment message means the target is not covered.

Various kinds of information can come from fusing the information transmitted by all platforms of a given type. An aircraft, or aircraft controller, listening to all the aircraft messages can create situational awareness that includes:

locations of friendly, hostile, and unknown aircraft

progress of friendly aircraft, committed to missions and targets, with respect to their planned route

locations of friendly and hostile ground forces

air defense weapons of both sides,

location of friendly air bases and alternative recovery points

A ground forces application, however, would ignore the more detailed aircraft-related information. Ground troops not part of the IADS need to know generally about the level of threat, and the location of friendly aircraft to be deconflicted. They have a more urgent need to know the position of other ground units near them, than, for example, an aircraft committed to a deep battlefield air interdiction mission supporting them far behind enemy lines.

Given that all subscribers to the net are able to supply such information, each platform can determine what targets are covered or not covered and will know whether other friendly aircraft or ground forces are available for support.

Architecture

Architecturally, JTIDS consists of a system of radios and a set
of conventions called links for managing the access of the multiple transmitters to the shared frequency system. Links 4A and 11 are being replaced by Link 16.

Words versus messages

JTIDS actually transmits "words" of data, which will form application-specific messages.

While the term "JTIDS message" is commonly used, the messages, which are architecturally at a higher level than JTIDS time slots, are actually defined by the NATO Tactical Data Information Link (TADIL) standards. TADIL messages are made up of digital words transmitted by JTIDS. It is U.S. usage, not NATO, to separate the link from the message specification; a given level link always uses the same TADIL level:

Link-16 uses TADIL J

Link-11 uses TADIL A

Link-4A uses TADIL C

Within a TADIL specification are three types of messages: fixed format, free text, and variable format. The fixed format messages are called J-series messages and are used to exchange tactical information. Free text messages are used for voice communications, while the variable format messages are user defined in length and content.

Radio components of the link types

Link 16 JTIDS radios operate in the L-band, which operate at line-of-sight ranges up to 500 nautical miles with automatic relay extension beyond line of sight. The true JTIDS messages and channels are separate, in frequency and channel spacing, from the TACAN position information.
[2]

Link 11 operates in the high frequency (HF) and ultra-high frequency (UHF) bands.

Link 4 is a netted, time division link operating in the UHF band at 5,000 bits per second. There are 2 separate "Link 4s": Link 4A and Link 4C. Neither, however, has the jam resistance of Link 16.

Medium access of the link types

Link 11

Link 11 is a centralized token system, with the Net Control Station polling the stations by PU number.[3] On completion of data transmission, the unit returns to the receive mode and the next unit is polled until all units have been polled. This cycle is continuously repeated.

It transmits in 24-bit words. Link-11 messages are called M series messages, under the TADIL A specification.

Communications security is provided by an external TSEC/KG-40 encryptor.

Link 4A

Link-4A still not fully distributed, but does use time division multiple access between an aircraft and a master operator. Within each timeslot, it uses command-and-response protocol to enable the operator to control multiple aircraft independently on the same frequency. Link-4A messages sent to the controlled aircraft are referred to as V series messages and messages received from the controlled aircraft are called R series messages.

Link 16

Link-16 uses a distributed rather than centralized control, among a time-synchronized set of stations. A station joining the net gets a JTIDS Unit Number (JU). Each JU is associated with a particular time slot in the epoch. As far as each JU is concerned, it listens, until its time slot comes up. If it has something to send, it does so, up to the maximum number of frames it can send in a time slot. The JU identifies the units and determines a preassigned set of time slots that designate when the unit transmits and receives data. Since Link-16 is the most current technology, its operation is discussed in more detail below.

Electronic protection and communications security

Spread spectrum with noise injection into unused frequencies, frequency hopping, and jitter in data start techniques combine to resist jamming and make JTIDS difficult to intercept.

Encryption adds additional protection against both unauthorized access to, or faking of, the information content. The encryption can be external to the JTIDS set, using TSEC/KGV-8A/B/C or an Embedded Encryption Device, TSEC/DS-101 or DS-102.

Link 16 Operation

Link-16 has the ability to form up to 128 multiple nets. The Link-16 system has 128 numbers used to designate particular nets (00-127); there is a set of 32 JU numbers within each net. Net 127 is reserved for the "stacked mode"; see below.

Basic time slot activity

Link-16 participants transmit data when the time slot corresponding to the JU number comes up in the cycle. The time division multiple access mechanism allows up to 32 transmitters, on each given frequency (or set of hopped frequencies_, are assigned "slots" in a 12.8 minute epoch. [4]When it is the transmitter's turn, it sends data or digital voice into the system, which can be up to 64 frames, each of which consists of 1,536 frames of 7.8125 milliseconds each. Tactical Digital Information Link, version J, (TADIL-J) define the frame formats. The participants repeat their messages whenever their time slot occurs, updating the information as needed. Repetition means that new participants will automatically begin to acquire information from the other participants.

Tradeoffs in use of time slots

The system can trade jamming and noise immunity against raw transmission speed. In "standard", the most reliable form, each slot begins with a pseudo-random delay at the beginning of each slot. Taken over a series of slots, this technique is a controlled method of introducing what is normally an impairment, phase jitter. Also in the standard mode, every other pulse in the message is repeated, which allows reconstruction using the Reed-Solomon forward error correction technique. Turning off redundant pulses makes the signal packed-2 format, and turning off the starting delay makes it packed-4.

To improve the receiver's ability properly to interpret the signal, each 6.4 microsecond pulse is "chipped" by 32 phase modulations (at 5 MHz speed) and transmitted at a one of 51 pseudo-randomly selected frequencies spaced 3 MHz apart. As many as 12 75-bit words can fit in a single slot.

Link-16 transmits data serially using 70-bit data words. During the transmit time slot, either three, six, or 12 data words can be transmitted. Three are sent when in standard mode, six in packed-2, Pulse redundancy can be disabled, which doubles the data rate into the packed-2 format. Eliminating the start-of-slot delay further increases the data transfer rate to the packed-4 data structure. In standard mode, the participant sends 1 word; 2 words in packed-2, and 3 words in packed-3.

Stacked net

Net number 127 is reserved to indicate a stacked net. A stacked net is formed by setting up the time slots so that they have the same
set, initial slot number, and recurrence rate. Loosely speaking, since the different stacked nets have different frequency hopping patterns, there are multiple stacked net 127s separated by a frequency-agile variant of frequency division multiple access.

One of the participants maintains the "clock" to ensure fidelity.

Deployment

In the list of platforms below, the early versions of a given type may not have had JTIDS, but the more recent ones have it installed.[5]

Navy

The Navy has faced rather significant challenges in mounting JTIDS antennas on ships, for a number of reasons. Their experience gives general considerations for any installation. Some antenna mounts were mechanically arranged to be on top of a mast, but were too heavy for the available masts. Others interfered with the signal patterns of the antennas of other systems.[6] The first antenna built for the purpose, with 16 signal elements in a dual cone structure, was made out of solid aluminium, was 63" in diameter, and weighted 400 pounds. This weight was vastly over the 100 pound limit for many locations. A contractor rebuilt it in metallized fiberglass, but this did not meet shipboard requirements.

One of the chief problems is that JTIDS also uses TACAN, so a TACAN antenna needs to be colocated. The 63" height did not permit mounting the TACAN antenna above the JTIDS proper, or as a piggyback mount, so it was again redesigned, with a lesser number of elements, to fit the available 43" diameter. Unfortunately, the 43" prototype was again in solid aluminium, which was easier for radiation technique, and another metallized fiberglass version had to be made to stay within the weight limit.